Translating material and method of manufacture



Aug. 10, 1954 s H. 2,685,728

TRANSLATING MATERIAL AND METHOD OF MANUFACTURE Filed Feb. 21. 1949 FIG.

lNl/E/V 70/? R. S. OHL BY 94m 7M A 7' TOR/VEV Patented Aug. 10, 1954 UNITED STATES rem oFFicE TRANSLATIZN G MATERIALA'ND METHOD OF MANUFACTURE Russell S. hl,,Fair Haven,"N. J., assignor'to'Bell Telephone Laboratories,

Incorporated, New

This invention relates to electrical translating materials anddevices and to methods of making them.

More particularly, theinvention relates to the manufacture of translating-devices having silicon portions which exhibit rectifying properties when combined with :limited area contacts.

The objects of this invention are to improve the electrical efliciency of translating devices employing silicon elements, to :improve the me- :chanical-connectionsof such elements to metallic backing pieces, to make practicable the formation of smallsilicon elementsona limited-area :of a backing member, and to simplify the techniquesof manufacture of such silicon translating devices.

.In the past few years, development of translating devices employing various semiconducmaterials has been greatly accelerated by :the :extension of the signaling art into the ultra-high frequency range where Waves. of

a few centimeters are-employed. At the present stage of the development, these translating 'devices are. usually "of the point-contact-type havingonezor morewfine wires mounted so their free :end's engage the surface of an element :having suitable rectifying properties. Silicon crystals containing significant traces of other elements which modify their electrical characteristics BX- hibit these rectifying properties and :provide a conveniently handled :element when mounted upon a'highly conductive backing piece.

' Recent developments have indicated that best results are attained with highpurity. siliconcontaining in excess of 99.9 per cent of theipurematerial with traces of significant impurities :in solution. 'Heretofore these metal mixtures have been cast into ingots in the manner set forth for example in R. S. -Ohl Patent 2,402339 issued June'25, 1946.

In producing rectifying elements from this type of material, the "ingot is cut into slalos' from which are cut wafers which-in turn are secured to highly conductive backing members. These silicon elements are then secured to aterminal block -'-by various bonds as exemplified in G; W. PickardPatent 1,137,714.0f April :27, 1915, wherein a metallic :cup containing :fusible metal is heated and the :rectifying elementplaced therein and secured by the freezing of the fusible metal; .Another commonly employed bond is formed by first .electrop1ating the .back .of .the

element with a metaL-suchvas nickel. or rhodium and vtl'ienlsoldering .the ,p'lated .elementito a suitable'base by means of any desirable solder.

When .the .elernentslare formed .by. either of the above-mentioned methods 'it is necessary that the siliconberseparatedrfromits ingot. This can .be done by crushing, -.or as previously described, by cutting and grinding pieces from ltheringot. When rectification ,is to be accomplished at very high. frequencies, .itis :desirable to .use thin, small area silicon wafers. The fabrication .of the bulk material into :suiiiciently small dimensions for these frequencies by the aforementioned meth- ,ods isextremely difficult. There is nocontrol of .theisize .of the elements produced "by crushing, .and.in the cutting .and grinding operation the crystallites of which the bulk material is -made up tend to chip out along .the cutting edges. .Tliusthelformation-of silicon rectifyingelements into dimensions of the order of less than one millimeter heretofore has been impractical.

Silicon has been deposited ontantalumichemicallylb y heating a (tantalum bodyin a controlled atmosphere :of hydrogen and silicon tetrachloride. lsucha process is disclosedsin J. :A. Becker Patent 2,438,892 .of April 6,19%];8. .This process coats the entire exposedarea of tantalum with silicon and cannot be conveniently limited to a particular portion of a tantalum body. Further, the :gases remaining after the deposition 'of the silicon .must be romoved under controlled conditions I prevent its +contamination.

The present method of forming silicon bodies for translating devices presents an advantage over the old processesiinthat it1providesaiv-means whereby very small silicon elements can "be formed either on a :small backing member or a limited :area :of a larger member. Further it produces-no by products :which might contaminate the silicon sand therefore the apparatus employed :is considerably simplified over that of the silicon deposition processes of the prior-art.

'The method of forming silicon'body portions onia ilimited sarea ofaa tantalum backing member in accordance with :this invention .producesa very active :P-tiZ-Pe silicon surfacediaving extremely high peak-back voltage when employed with a limited area contact as a rectifier, and both P and N-type silicon elements with up to 50 volts peak-back voltage, When combined with tungsten point contacts, have been produced. Briefly, this method comprises placing particles of high purity silicon on a prefabricated tantalum back member of any size and of various forms, and heating the tantalum to melt the silicon. The process is carried on a high vacuum of the order of 10 millimeters or better. The tantalum is first heated sufficiently to start the fusing of the sili con, and on this initial fusing, which does not occur until the temperature of the tantalum is well above the fusing temperature of silicon, the tantalum is cooled to below the fusing temperature of silicon to stop any reaction between it and the silicon. The tantalum is then reheated slowly until the silicon fuses uniformly over its entire surface. Since the second melting does not require as high a temperature as was employed in the initial melting, no chemical reaction occurs between the silicon and the tantalum. Upon cooling, the silicon crystallizes and the surface is ready to be employed in a translating device.

It is to be understood the designation P-type silicon used throughout this psecification refers to silicon which passes current readily only When it is positive, while N-type silicon passes current readily only when the silicon is negative.

Other features and advantages of the invention will be discussed more fully in the following detailed description which is more clearly and fully understood when read with reference to the accompanying drawings in which:

Fig. l is a plan view of one form which can be employed in the construction of a rectifying element in accordance with this invention;

Fig. 2 is a cross-section of the element of Fig. 1 taken along the line 2-2 of Fig. 1;

Fig. 3 is an elevation of a holder for an element such as that disclosed in Fig. 1 adapted for use in a translating device of the type disclosed in Fig. 6;

Fig. 4 is a plan view of another form in which the rectifying element of this invention can be made;

Fig. 5 is a cross-section of the element of Fig. 4.- taken along the line 55;

Fig. 6 is an elevational View partially in crosssection of a translating device including one of the rectifier elements of this invention; and

Fig. 7 illustrates the apparatus used in applying the silicon to the metal backing.

Limited area rectifier elements constructed in accordance with this invention may assume vari- 0115 forms and sizes all of which include a high purity silicon body fused on a tantalum body which forms the back contact for the device. The structure of these devices can most readily be understood by reference to the process by which they are manufactured.

The tantalum back contact for the silicon rectifier element must be of extremely high.

purity since any impurities which might be present would contaminate the silicon by combining or going into solution with it during the manufacturing operation. The element shown in Figs. 1 and 2 employs a dished tantalum body formed from sheet tantalum and that of Figs. 4 and 5 employs a tantalum rod dished on its end. In the operation of forming these and other types of back contacts, the high purity tantalum is subject to contamination from handling and Lil from the tools employed. Therefore, it is necessary to clean the tantalum as a preliminary step in the forming of the rectifier element. As an example, where a steel punch and die are employed to form the tantalum body as in Fig. 1, it is first cleaned in chloroform to remove the grease, which might be deposited on it during handling, then dried and heated in diluted sulphuric acid to remove any traces of iron which might have adhered to the body from the die and punch.

Particles of high purity silicon are then placed in the clean tantalum cup. Most satisfactory results have been obtained by employing finely divided silicon supplied by E. I. duPont de Nemours and Company having a purity considerably in excess of 99.9 per cent produced by pyrolytic reduction of silicon tetrachloride on zinc at high temperatures. The elements in a typical sample of this material include:

Per cent Silicon 99.95 Iron .03 Calcium .005 Sodium .005 Copper .001 Manganese .001 Magnesium .001

In some samples traces of zinc, silver and aluminum have been found, all of these in quantities less than .03 per cent.

The element is then placed in an apparatus such as that shown in Fig. '7 and the tantalum is heated to fuse the silicon. This apparatus comprises a high vacuum system including an evacuated chamber 40 containing a vacuum of the order of 10" millimeters or better which is established and maintained by a high vacuum pump represented by the labeled block. A quartz plate 4| is mounted within this chamber on a platform A2 and supports a graphite disc 53 which in combination with a surrounding high frequency induction coil 44 (shown adjacent the disc for purposes of clarity) serves as an induction heater. The tantalum body 45 is placed on this graphite disc and heat is applied by the application of high frequency current to the induction coil.

In order to produce a satisfactory rectifying element the heating process must be carefully controlled. Under the application of excess heat the tantalum and silicon combine chemically to form a brittle non-rectifying compound which obviously is extremely undesirable for the purpose of this invention. In order to fuse the silicon particles it is necessary to raise the temperature of the tantalum backing to about 1600 C. which is well above the fusing temperature of silicon. When this fusing begins an absorption of heat is evident indicating an endothermic chemical reaction occurring between the silicon and tantalum. If this reaction were allowed to continue, it would destroy the element for rectifying purposes as pointed out above. Therefore, the power is removed from the heating coil as soon as the silicon beg-ins to fuse thereby cooling the element and preventing more than a very limited reaction between the tantalum and silicon. When the temperature of the element is cooled sufficiently to stop the reaction, this has been found somewhat below the fusing temperature of silicon, heat is again applied to the tantalum body and it is slowly brought up to the fusing temperature of silicon, about 1420 C., and

completely fused it is slowly cooled to crystallize it into a very highpuritybody having "a smooth surface wliichieiililbits'excllentrectifying properties.

Thesilicon point contact rectifiers of :the prior 'afitliavebeen limited iniheinpeabback voltages to ar ange extending fromabout 2 to 115 volts. Rect ifiers-"produced by this method exhibi'bmuch higher peak bacl voltages withtungsten point contacts, 50 volts being not at all uncommon for both P and N-type silicon, and some P-type silicon elements having peak-back voltages of up to 200 volts with tungsten point contacts have been produced by employing selected particles from the lots of material supplied by E. I. duPont de Nemours and Company an fusing them to the tantalum back contact in the manner described above. These particles which can be formed into high peak-back voltage material are selected by their appearance which differs from the crystalline appearance of the remainder of particles in the lot as received from the supplier in that their surface appears sintered and appears to be coated with a dirty brown film.

Referring now to the drawing, the rectifying element of Figs. 1 and 2 which may be utilized in various forms of translating devices and particularly point contact rectifiers, comprises a tantalum body II having a peripheral flange l2 included for convenient mounting of the element and a dished portion l3 providing a receptacle for the silicon rectifying body It. The small area in which the reaction between the silicon and tantalum occurs, referred to in the preceding description of the process, is represented by the shaded area I5 in the cross-section view of Fig. 2 (greatly exaggerated in size for purposes of clarity). This element can be conveniently employed in the conventional type of point contact rectifier disclosed in Fig. 6 by mounting it on the metallic contact holder l6 and securing it so that the dished portion 13 rests in the bore I I by some convenient method such as spotwelding the flange l2 to the lip I8 on the end of the holder.

Fig. 4 shows a modified form of rectifier element which may conveniently be employed where it is desirable to have a silicon rectifier body l4 mounted on the end of a rod 20. This structure might be of particular advantage in the construction of a rectifier for use in wavelengths of a few millimeters since it avoids the necessity of fabricating the bulk materials into satisfactorily small dimensions or machining a chemically formed unit to reduce the area of the silicon rectifying body. Units for these frequencies must often have dimensions of the order of less than one millimeter and are therefore difficult to handle and to form by the processes known in the prior art while no such limitations are imposed on the silicon body of Figs. 4 and 5 and therefore they can quite conveniently be produced and mounted in the small spaces which are required for millimeter wave reception.

An assembled rectifier of the point contact type which might employ the rectifier element of this invention is illustrated in Fig. 6. This rectifier comprises a cylindrical metallic shell or housing 22, a crystal assembly, and a contact assembly. The metallic cylinder 22, which also serves as an electrical shield, may be made either of steel or beryllium copper and may be given a thin coating of tin. The crystal assembly is that sho'wn it'l -Fig. 3 and comprises a' silicon "surface ion the tantalum body -Il 'secure'd' to the solid co'nductive cylinder lli. The spring coiitactwzire assembly the rectifier -comprises a metallic pin 23, an insulating-cylinder 24 molded' around the pin 23 or otherwise permanently affixed thereto, "and a short length of tungsten --con'tact wire 25 secured in conductive relationship to one end of 'the pin 23. Theorystal supporting cylinder P6 and the insulating cylinder 24 are 'inserted in opposite "ends of the housing '2 2' and are advanced towards each other until the wire 25 engages the surface of the silicon element to form a limited area rectifying contact with the silicon. These elements are maintained in their proper relationship by a force fit between the cylinders I6 and 24 and the housing 22.

What is claimed is:

1. An element for use in rectifying devices exhibiting peak-back voltages of the order of 200 volts when employed in combination with a limited area tungsten contact, comprising a tantalum member, a layer of a compound of tantalum and silicon in intimate contact with said tantalum member, and a layer of high purity silicon in intimate contact with and superposed on said layer of tantalum and silicon, said silicon being fused and crystallized in a vacuum from particles having a purity in excess of 99.9 per cent and a brownish sintered appearance.

2. The method of making material for use in electrical translating devices which comprises mechanically placing solid particles of silicon having a purity in excess of 99.9 per cent on a tantalum surface, heating the tantalum in a vacuum to a temperature above that at which tantalum and silicon react until said silicon commences to fuse, cooling said tantalum to stop any chemical reaction between it and said silicon, reheating said tantalum to completely fuse said silicon, and cooling said tantalum to crystallize said silicon.

3. The method of making material for use in electrical translating devices which comprises shaping a tantalum base to form a cup surface on one side, mechanically placing solid particles of silicon having a purity in excess of 99.9 per cent in said cup portion, mounting the silicon and tantalum in a vacuum, heating the tantalum to a temperature above that at which tantalum and silicon react to cause an initial fusing of the silicon, cooling the silicon to stop any chemical reaction between it and the tantalum, slowing reheating the tantalum to a temperature sufficient to complete the fusion of the silicon, and thereafter slowly cooling the tantalum to crystallize the silicon.

4. The method of making an element for use in electrical translating devices which comprises, mechanically placing solid particles of silicon having a purity in excess of 99.9 per cent on a body having a tantalum surface, heating the body in a vacuum to about 1600 0., cooling the body to below the fusion temperature of silicon upon the initiation of an endothermic chemical reaction between the silicon and tantalum surface to stop said reaction, reheating said body to about 1420 C. to completely fuse the silicon, and cooling said body to crystallize the silicon.

5. An element for an electrical translator comprising a tantalum base member, a layer of a compound of tantalum and silicon in intimate contact with said base member, and a layer of high purity silicon in intimate contact with and superposed on said layer of tantalum and silicon.

6. The method of making an element for an electrical translator which comprises placing silicon on a tantalum surface, heating the tantalum in a vacuum until said silicon commences to fuse and form a compound of tantalum thereby establishing an intimate heat transfer relationship between said tantalum and the remaining unfused silicon, cooling said tantalum to stop any chemical reaction between it and said silicon, maintaining said tantalum above the fusion temperature of silicon and below the temperature at which tantalum and silicon react, and cooling said tantalum to crystallize said silicon.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,826,955 Ruben Oct. 13, 1931 2,235,051 Thompson Mar. 18, 1941 10 2,438,892 Becker Apr. 6, 1948 2,452,577 Kotterman Nov. 2, 1948 

1. AN ELEMENT FOR USE IN RECTIFYING DEVICES EXHIBITING PEAK-BACK VOLTAGES OF THE ORDER OF 200 VOLTS WHEN EMPOLYED IN COMBINATION WITH A LIMITED AREA TUNGSTEN CONTACT, COMPRISING A TANTALUM MEMBER, A LAYER OF A COMPOUND A TANTALUM AND SILICON IN INTIMATE CONTACT WITH SIAD TANTALUM MEMBER, AND A LAYER OF HIGH PURITY SILICON IN INTIMATE CONTACT WITH AND SUPERPOSED ON SAID LAYER OF TANTALUM AND SILICON, SAID SILICON BEING FUSED AND CRYSTALLIZED IN A VACUUM FROM PARTICLES HAVING A PURITY IN EXCESS OF 99.9 PER CENT AND A BROWNISH SINTERED APPEARANCE. 